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How to obtain true altitude from indicated altitude, temperature and QNH?
maybe it's a little impolite to ask but I got stuck with knowing bunch of formulas and not understanding how to use it:
Can you help me on understanding how the correct answer 7300ft came out of the following question:
Given the following information, what is the true altitude? (rounded to the nearest 50 ft) QNH: 983 hPa;  Altitude: FL 85; Outside Air Temperature: ISA - 10° (1,00 P.)

My attempt based on the fomula from here:

$A_T$ is our estimated true altitude, in feet
$\Delta_{ISA}$ is the difference above ISA, in °C
$A_I$ is our indicated altitude, in feet
$A_{Ik}$ is our indicated altitude, in thousands of feet

$A_T = (4\times A_{Ik} \times \Delta_{ISA}) + A_I$

$A_{Ik} = 8.5$
$\Delta_{ISA} = -10° - ( 15° - (8.5 \times 2) ) = -10° - ( 15° - 17°) = -10° - (-2°) = ISA-8$
$A_i = 8,500$

Therefore,
$A_t = ( 4 \times 8.5 \times (-8) ) + 8,500 = ( 34 \times (-8) ) + 8,500 = -272 + 8500 = 8288$
So my incorrect answer is 8288'. The correct is 7300' :(
I also realize I did not use QNH in my calculations.

You need to correct your pressure altitude (FL85) for the barometer setting QNH first before you apply the temperature correction.
I assume a 30ft / hPa offset here:
$\textrm{barometric altitude} = \textrm{pressure altitude} + (\textrm{QNH} - 1013.25)\cdot 30 = 8500 -907.5 = 7592.5 \textrm{ft}$
Based on the pressure difference between sea level and your altimeter's measurement, the estimated altitude is 7592.15, assuming an ISA temperature profile.
Now, due to the fact that the actual temperature is lower, the atmosphere shrinks, and you end up flying lower.
Note that I will use -10° C for $\Delta_{ISA}$, because that was given in the question:

Outside Air Temperature: ISA - 10°

Now applying your correction formula yields:
$A_T = (4\times A_{Ik}\times\Delta_{ISA})+ A_I = (4 \times \frac{7592.5}{1000}\times -10)  + 7592.5 = -303.7 + 7592.5 = 7288.8 $
Rounding that to the nearest 50 ft results ins 7300 ft.
This is an approximation which is a bit on the low side. But underestimating true altitude is usually safer than overestimating.

WOW! Thank you, DeltaLima, for your comprehensive answer and for editing my post with proper formula formatting (it’s much easier to read). That helped me, and that will help people looking for the answer in the future.

SUCH A GREAT ANSWER ! Helped me a lot understanding those problems !! Thanx


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How would one perform a more thorough preflight of the engine on a single-engine airplane without easy access to engine compartment?
On a lot of the Cessna 172 and 182 airplanes I have seen, there isn't an easy way to look at the engine in the engine compartment and see all of the components during a preflight. In fact, usually the only thing you can really do to see the inside of the engine compartment is to peer through the tiny oil/fuel sump door.

The POH for a 1978 Cessna 172N in the Preflight Checklist simply says:

Engine Oil Level -- CHECK

Before first flight of the day and after each refueling, pull out strainer drain knob...

Propeller and Spinner -- CHECK for nicks and security.

...

Carburetor Air Filter -- CHECK for restrictions by dust or other foreign matter.

...

So I know that while it's not a requirement to see the components of the engine and look at the engine compartment, it seems prudent to add something to your preflight that might help give you an indication if you should investigate further before attempting the flight.
How do you more thoroughly preflight the engine of your aircraft when you can't access the engine compartment without completely removing the cowling?

You're right that this does limit things a bit. I don't fly a 172 too often but when I do I just pay special attention to the cowling floor, using a torch if necessary, checking for pooled oil or debris. Combined with thorough engine run-ups you should pick up any major problems.

These issues don't pop up that often that you would need to include them on a preflight check. Those aircraft/engines are reliable enough that any issue can usually be determined by things like:

Excessive oil use
Oil Temperature / Pressure Changes
Rough running or improper behavior during run-up

That is why you do a run-up in these aircraft that checks many systems, so you can spot those kinds of issues. But the reality is that even if there is an oil leak or missing safety wire, those issues are reviewed (in detail) and fixed in the annual inspection. The aircraft is reliable enough to not worry about it between inspection intervals. For rental/hire aircraft this inspection happens more often, every 100 hours.
Some of the issues may be caught during oil changes as well, which happens every 50 hours (depending on manufacturer recommendations).
Let me give you an example...
I was doing some recurrent training and selected a Piper Cherokee. I was doing the preflight and you can open the cowl on those aircraft. I noticed a significant oil leak behind the controllable pitch mechanism which my instructor and I decided not to fly until the mechanic looked at it. When the mechanic did, he said that it probably needs to be rebuilt but to go ahead and fly it anyway, just make sure that the oil was full prior to flight.
On my 177 we had been flying it all year, when it went into annual it was found that the piston controlling the constant speed propeller was way out of spec and that the hub had corrosion. The entire prop had to be replaced. We were flying it and it was behaving just fine, but could have failed at any time.
The point is that the issues that will stop your engine from running or make your prop run away are usually not visible anyway. Keep it topped up with oil and pay attention during run-up and you should be fine. Usually I take a flashlight and peer around in the compartment as best I can to make sure nothing has taken up residence in the engine, but aside from that, there isn't much else to do preflight.

On a Cessna you focus on what you can see. I have a powerful flashlight I use to look in the hole in the cowling, which would be good enough to catch some problems. I also have a good look under the cowling to look for any drips.
The other thing I use is my nose, I take a good sniff or two through the oil door of the cowling. There are failures which you'd never detect from a visual inspection, for instance electrical components when they fail have a distinctive smell. I once detected an alternator failure this way when visually it looked fine. If you have an oil leak or a fuel leak you'd possibly smell that too.

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What are the registration requirements for US aircraft?
Where can the requirements for US aircraft registration be found?
What is the penalty for flying an aircraft without a valid registration?
What should someone do if they believe an aircraft to be operating without valid registration, and how could they determine this?
Somewhat related to this other question:
What is the pilot's obligation to verify a registration has not expired?

Where can the requirements for US aircraft registration be found?

The registration procedure is outlined in FAA Form AFS-750-94, Information to Aid in the Registration of U.S. Civil Aircraft. See also the full list of FAA registration forms.

3. The owner of an aircraft must apply to the Aircraft Registration Branch, P.O. Box 25504, Oklahoma City, Oklahoma 73125-0504, to obtain a certificate of registration. The applicant must submit an Aircraft Registration Application, AC Form 8050-1, accompanied by evidence of ownership and a $5 registration fee.
Requirements for registration of aircraft are contained in the Federal Aviation Regulations, Part 47, “Aircraft Registration.” (For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402.)

Basically, the registrant (whether an individual or a corporation, trustee, etc., all of which are defined in Part 47) must be able to prove legal ownership of the airplane.

What is the penalty for flying an aircraft without a valid registration?

The violator would be subject to any of the standard FAA enforcement actions: administrative action (a "Warning Notice" or "Letter of Correction" which goes on your record), reexamination ("709 ride"), certification action (suspension or revocation), or civil/criminal penalties. The exact penalty would likely depend on the circumstances: whether the violation was intentional and the degree of the pilot's negligence under the FARs.

What should someone do if they believe an aircraft to be operating without valid registration, and how could they determine this?

The FAA's website recommends reporting FAR violations by calling the FAA's toll-free telephone number, 1-866-TELL-FAA.

Sounds like you want to read 14 CFR part 47 - Aircraft Registration.
As for most penalties with the FAA, it can vary wildly, from a warning if it is a minor or technical infraction, potentially all the way to lifetime loss of license and criminal charges if done deliberately and maliciously.

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Why do winglets reduce maneuverability (of fighter jets)?
In this answer Ethan makes the following statement concerning winglets on fighter jets:

[Winglets] reduce maneuverability which is why you don't see a winglet on a fighter jet

I could image that adding more weight at the end of the wings increases inertia, reducing maneuverability, but I'm not sure if this is the only reason.
Perhaps there is some aerodynamic phenomenon at play as well?

Your observation about roll inertia is correct: Winglets add proportionally more roll inertia for their small increase in L/D, and this increase exists only for the higher range of lift coefficients. At low lift coefficients (think cruise), when the induced drag is low, they add more friction drag and reduce the L/D overall.
There are three aspects to maneuverability:

How quickly can you accelerate into a roll? Here the added inertia of winglets reduces roll acceleration
How high is the maximum sustained turn rate? This is determined by the maximum lift at zero sink rate, and here winglets are helping. Especially when mounted to low aspect ratio wings.
How high is the maximum turn rate? Now only lift counts, and the increased drag is compensated by an increased sink rate. Again, winglets help a little, but less than with the sustained turn rate.

Now look at the time any fighter will spend around 1g (hint: Something close to 100%) and how much is spent at high lift coefficients, like when turning tightly in air combat. Winglets would help in the turn (and I think Ethan's opinion is wrong), but be a source of drag for the rest of the mission. Adding winglets would require to increase the fuel volume to prevent a reduction in flight time and range.
Normally, winglets are only used on highly loaded, high-aspect ratio wings when their L/D at a rather high lift coefficient needs to be pushed up even more. Highly maneuverable configurations are less concerned with single-percentage increases in L/D for a small part of the flight, and in order to improve turn performance, increasing wing span is much more effective. But that increases roll inertia and roll damping, so fighters use low-aspect ratio wings and compensate their higher drag in a turn with more powerful engines. Note that the trend in fighter aspect ratios went down with the improving thrust-to-weight ratio of jet engines.
Now that is the reason why you don't see winglets on fighter jets: They would lower L/D for most of the flight. To perform the same mission, the fighter without winglets can be made smaller.
But let's make no mistake: Low aspect ratio configurations gain the most from winglets: They were essential on the Hermes re-entry vehicle project to give it enough L/D for a successful flare.

Can you clarify why fighters don't have winglets but passenger jets do? It seems like the same analogy of spending most of the flight in cruise would apply. I'm aware we have discussed this before.

@fooot: Fighters have less wing loading and lower operating altitudes than airliners, so they fly at lower lift coefficients. This makes induced drag less pronounced, so doing anything to lower induced drag will have less payoff. In other words, they carry around much higher lift reserves than airliners which prefer to fly close to their coffin corner. If induced drag needs to be reduced, a bigger wing span would be most helpful.

@PeterKämpf: but then most, if not all, modern high-performace gliders use winglets.

@MartinArgerami: Yes, in the span-restricted competition classes they make sense. The rest is psychology.

@PeterKämpf: are you really claiming that the ASW22, the ASH25, the JS-1, the ASH31, the Concordia, the ETA, and others all have winglets because of "psychology"?

@MartinArgerami: Yes, I do. Absolutely! Did you ever fly in a glider competition?

Yes, in Argentina, maybe 10 in total between regionals and nationals. I never won the nationals but I was competitive ( I did win several regionals and  several days at the nationals over different years).

@MartinArgerami: Then you know how a pilot is affected by a perceived disadvantage. It makes them take disproportionate risks to "compensate", and most times they end up worse even though their glider had no objective disadvantage. Next time, they will not start without that wonder device that makes the others so much better, even though the disadvantage existed only in their mind all the time. I call it the "cult of winglets".

I know the mentality perfectly, because for more than 10 years I was in charge of the Argentinean handicap system. And almost every pilot felt the system was against him. Or, like a friend used to say (in now club gliders): "we all know that the Jantar 2b and the ASW20 are basically even; that the ASW20 and the LS4 are pretty even; that the LS4 and the Jantar Std are even, and that the Jantar Std and the Cirrus Std are even. In conclusion, the Jantar 2b and the Cirrus Std are even".

Still, modern gliders have greatly improved in handling; and my understanding is that on his time Masak did conduct measurements with different types of winglets.

The effects of winglets on the maneuverability of aircraft is not straightforward, with different effects on various maneuverability parameters.

In the simplest sense, the winglets have the effect of increasing the aspect ratio of the wing. This results in lower roll angular acceleration as a higher moment of inertia needs to be overcome before the movement begins.
The roll rate (at maximum aileron deflection) is slightly greater with winglets, as they increase the local span loading near the wing tips, thereby augmenting aileron effectiveness. This means that smaller aileron deflections are required for a given rolling moment; meaning less drag for a given roll rate and a higher maximum roll rate.
As the winglets increase the L/D ratio, the climb performance is improved.

There are other reasons for the absence of winglets in combat aircraft, such as:

Winglets are optimized for a particular configuration of wing, flight speed and profile and are usually ineffective, and may even have adverse effects on aircraft performance in other situations. For example, the winglet increases profile drag while reducing induced drag; in conditions where profile drag is a major contributor to the total drag, this is a disadvantage.
The winglets will have disastrous effect in supersonic flight due to the added drag.
The thin winglets are prone to flutter problems.
Winglets will add considerably to the radar signature from the most critical angles.

An unsung property of winglets is that they improve yaw stability, which can actually reduce drag, especially for designs which require yaw damping.
Going back to the days of the Sopwith Camel, fighters benefit from instability to turn more effectively.  While today's fighters have computers to make them safer, the Camel was very challenging to fly.
Contrary to some opinion, winglets are not a "drag disaster" and were very sucessfully used on the XB-70 design (folded down) to increase the lift coefficient of the delta wing, allowing it to fly at a drag saving lower angle of attack.

XB-70 might not be the best example here. It used very different aerodynamic theories, essentially riding its own shock wave for lift at supersonic speeds.

It does display the potential for even a supersonic bomber/tanker/cargo/passenger plane to use them.  What I like about them on the XB-70 is that they were designed to be movable in flight, also helping the delta generate more lift at lower speeds by increasing lifting area when rotated back to horizontal.

But I am pursueing the idea that a solid fin, even with its weight, may be more beneficial than an area of drag producing turbulence.  Check out the blue fin tuna!  See where its fins are, particularly along its back (dorsal) area.

Not sure about the little spikes, but I would guess the bigger fins on the blue fin tuna are used like the daggerboard on a sailboat.

The thing about the XB-70 is that having the wing tips flat is the more aerodynamic configuration, both for the amount of lift and the amount of drag created while in normal flight. At high speeds the wing tips fell down to contain the high pressure of the shock wave, which with the flow going parallel to the underside of aircraft gave almost free lift for drag that had already been created.

@XRF -- Re "Not sure about the little spikes, but I would guess the bigger fins on the blue fin tuna are used like the daggerboard on a sailboat." -- sailboats operate on the interface betweeen two media which creates radically different dynamics.  You'll never see a daggerboard, keel, or many other sailboat features on a one-media craft such as  a submarine --

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Where do I start learning about Avionics programming?
I am a student in his last semester going for a B.S. in Computer Science. I would like to eventually transition into the space exploration industry (NASA, SpaceX, Boeing, Lockheed Martin, etc). I know nothing about avionics and i would like to learn more about the subject, specifically the software side. I have found little information online. Please give me reference material (books, websites, etc.) to learn about this area. Your point of view and additional information is strongly welcomed.

I don't have any books/materials or more concrete information, so I'll post this as a comment not an answer, but I'd strongly suggest brushing up on your low-level programming if you're serious about getting into this area.

@JonStory: Definitely C and C++. Though things like electronic flight bags are probably done with Java and even HTML and JavaScript.

I'm not convinced Java gets a huge amount of use in an airline - the JVM introduces too much uncertainty to be used in a Real Time application: but if you're competent enough in C/C++ to get this kind of job, you can do Java in  your sleep.

@JonStory: Hard real-time is always C or non-hosted (limited subset of) C++. But electronic flightbag is an application for showing charts, checklists and doing calculations. It has no real-time requirements and normally runs on standard notebook or tablet, though they are getting built in in new planes like A380. So it can be in Java or even HTML+JavaScript.

@JonStory: By the way, if you mention airline, most of IT they do probably is in Java and HTML. Because airlines don't do avionics. They do ticket reservation systems and flight planning applications and the electronic flightbags and none of that needs real-time. By the way at my former employer one team did system for controlling airport lights (approach, runway, taxiway, stopbars etc.) and even that was in Java. Because it was mainly GUI for the tower to switch the lights and that did not need any real-time either.

tablet EFB apps that receive data from TIS-B and weather may not be hard real-time, but it would be wise to display the information fairly quickly.

Sorry yeah, I meant Aviation generally not specifically airlines. EFBs are one thing, but I wouldn't count one as 'avionics' in any way.

http://programmers.stackexchange.com/a/153267 is a good read on the subject.

@rbp: "real-time" has very specific meaning completely independent of "fairly quickly". Java is fine for "fairly quick". On the other hand "real-time" systems are often not particularly quick; they just have to be absolutely free of any unpredictable delays (which makes GC out of question; usually even any dynamic allocation at all is).

thanks, i've been programming hard real-time for 35 years, and I chose my words carefully

Embry Riddle has a degree program that is exactly what you want.

How is it that this part “Please give me reference material (books, websites, etc.) to learn about this area. ” was flagged altogether as primarily opinion-based? Maybe it was edited later...?

Ok, I was posting this as a comment but was becoming ridiculously long...
I worked for most of my professional life in fields that are somehow related.
I can see that there are three areas of study that will help you a lot here.
First: programming languages. Knowing a language used in the industry is a plus.
Now I am working in the automotive/transportation industry; I am not working on safety critical material, so we just use standard C/C++. Even at that level a good knowledge of low-level programming and software (how to write a simple driver, how to interact with a device using ioctl, etc..) are very useful.
Before that, I worked on a field related to programming languages and compilers, especially on the simulation and analysis of concurrent code. Among the others, I briefly interacted with people working for Airbus (professors that are in academia, but help to build real software).
They still see and use a lot of Ada out there, so.. knowing that is good :) I also know for a fact that Java is also used, together with special tools for verification of the code.
Which bring us to the second: everything related to the verification of software, both from a theoretical and a practical point of view, will help you hugely in this industry. Study and practice with runtime monitors and model checkers, at least. Learn about various static analysis techniques, like abstract interpretation. Take classes about that, if you can at your school.
And third: study and practice with embedded systems and with the (more rare) real-time OSes and how they work (and how you do program them).
Get yourself an arduino or a netduino (not a raspberry PI - too high level, too many resources there!) and practice with them. Lear about uboot.

Verification, test, validation, the formal parts of software engineering are most important. If the code is safety critical - which it will be - you are in an entirely different league, up there with satellites. You need experience in really careful, excruciatingly pedantic requirements engineering. Avionics projects I have worked in were done in big international teams, so you need experience in this too.

And in your CV you should have plenty of the requirements management and version control tools; Doors, Serena, whatever. Subversion, CVS. It doesn't really matter which, because every company has its own variation, you just have to be used to the idea and willing to submit to the discipline.

I suspect the only answer is "on the job".
Learn both C and C++ and some embedded programming and try applying for any junior positions in companies doing avionics you'll find.
Mobile phones can't really be considered embedded any more, but you should be able to find a course in robotics or cybernetics (industrial controllers) or an internship involving those. An internship doing consumer electronics could help too.
And don't forget the aircraft manufacturers don't do avionics themselves but either buy standard modules from vendors like Garmin or Honeywell or subcontract it to many consulting companies. And those companies often do other kinds of embedded (automotive, industrial, etc.) too.

Notes: I remotely know someone who did avionics and I don't think he specifically aimed for it; he was just good C programmer and applied for a job. I myself did some automotive, again just applied for (junior; it was right after graduation) C/C++ job in a consulting company that was originally mobile and was shifted to automotive as the original project didn't really get going.

+1 for your note: I am working in a related field (automotive/transportation) and just applied for it. I had good C/C++ skills and that was what it took.

For avionics, the following will be essential just to get your CV considered:-

Good skills with C
Control theory, particularly state space and beyond
Related to control theory, a strong foundation in maths
Experience with hard-real-time processing
Experience working to coding standards
Familiarity with review processes

The following would also be useful:-

Experience of Ada, and possibly C++
Some knowledge of safety-related procedures (DO-178B, MISRA, etc.)
Knowledge of test-driven development
Experience of common RTOSes such as VxWorks
Experience of low-level programming on microcontrollers

With a BS (not BEng) in Computer Science (not electronic engineering) then you may be at a disadvantage on some of this.  You MUST choose your projects to get the right experience you want to take to employers.
Of course you're only at the very start of your working life though.  A degree is only the start, and any employer hiring graduates will know that you need training up.  A good employer will look more at generally how bright you seem to be and how well they think you can pick up new skills.  If you're playing with tech in your free time, this is probably a good thing to bring to interviews too.

I have a friend who works in Avionics software. Including space projects.
He got into it by applying for a job at companies that work in that field or similar, before then he didn't have any special avionics training or experience.
Avionics work tends to make more use of programming languages that are not mainstream. I think there's a lot of embedded C (so general C skills may be useful) but also processor-specific assembly and languages like Ada (perhaps more so for defense avionics).
You are very likely to be targeting a real-time operating system (RTOS) or microcontroller. So knowledge of these may be an advantage. You might need to write device drivers and port them to new platforms (e.g. from one microprocessor to another). You might need to program microcontrollers (e.g. 8051) in assembly or C.
It is likely that you would encounter a much higher stringency in terms of writing and/or using unit-tests, test-harnesses, coverage tools, integration-testing and painstaking formal documentation and review of everything.

Ada probably has more traction in civilian use than military nowadays, perhaps military contractors still have a knee jerk reaction against the mandate, 20 years late... Airbus and Boeing are big users, as are railway systems and air traffic control. SPARK is gaining traction too.

Learning C and C++ has already been suggested, and I would also add the Ada programming language to that list. A lot of avionics software is written in those three programming languages.
While learning C/C++, learning how to code properly according to coding standards for aviation software may be helpful, too.
I recommend reading:

Lockheed Martin JSF coding standards
ESA C and C++ coding standards

If you would like to play around with actual avionics programming, you may be interested in getting an open source quadrocopter (such as the one from AeroQuad) as a toy and experimenting with its software.

This. Play around with open source avionics, ground control systems, stabilizers, trackers, that kind of thing. Ardupilot, Mission Planner, UDB/MatrixPilot, etc.

I don't work with avionics, but I have worked with remote sensing in the context
of Earth Sciences.
I think these materials could be of some help in addition to what others have suggested, esp. if you happen to be interested in the science aspects (sorry the answer may be somewhat off-topic if you are not).

Programming
In addition to C, Fortran is still used a lot for implementing various
algorithms and models. For smaller tasks and visualisation,
people often use Python (numpy, scipy, pandas, matplotlib) and R (R is an excellent statistical programming language, something you would use instead of Excel for serious things).
With regards to C, nobody seems to mention that secure coding is really
necessary for such high-stake applications. Seacord, 2013 is a great
book for this purpose.

Chapman, Stephen J. Fortran 95/2003 for scientists and engineers. McGraw-Hill, 2008.
Seacord, Robert. Secure Coding in C and C++ (2nd Edition) (SEI Series in Software Engineering). Addison-Wesley Professional, 2013.
Matloff, Norman. Art of R Programming. No Starch Press, 2011.

Physics
Good fundamentals in physics are certainly vital in space exploration.
A lot of measurements done in space have to do with radiation,
so I'm adding a reference to that.

Physics I: Classical Mechanics,
Physics II: Electricity and Magnetism,
Physics III: Vibrations and Waves
video lectures by Prof. Walter Lewin (MIT OCW)
Halliday, David, Robert Resnick, and Jearl Walker. Fundamentals of physics extended. John Wiley & Sons, 2010.
Petty, GW. A First Course in Atmospheric Radiation (2nd Ed.). Sundog Publishing, 2006.

Mathematics
Calculus is probably the most important thing, then linear algebra.
Many problems in design and physics have to do with statistics and optimization.

Linear Algebra video lectures by Prof. Gilbert Strang (MIT OCW)
Wilks, DS. Statistical Methods in the Atmospheric Sciences. Academic Press, 2011.
Probabilistic Programming & Bayesian Methods for Hackers (available online)
Kruschke, J. Doing Bayesian Data Analysis, Second Edition: A Tutorial with R, JAGS, and Stan. Academic Press, 2014.

Unix/Linux
Unix operating systems are used a lot in applied sciences, embedded
systems and supercomputers.

Kerrisk, M. The Linux Programming Interface: A Linux and UNIX System Programming Handbook. No Starch Press, 2010.
Lucas, MW. Absolute FreeBSD: The Complete Guide to FreeBSD. No Starch Press, 2007.
many other books, e.g. ones published by O'Reilly

Some daily reading: phys.org/space-news/ (-;

I highly suggest looking into a software analysis course and learning a proving system such as PVS.  I went back to graduate school after 14 years in various software industries, and the software analysis course I took has easily been the most eye opening.  It wouldn't have made much sense when I was in school, but after working for many years the material clicked.  It was never a subject I felt I would enjoy, and it can be a bit dry, but once you get it, you'll never look at programming quite the same way,

From a little search on the net I have found this book: Developing Safety Critical Software: A practical guide fro Aviation Software - Rierson.


text:
What is this white smoke like thing, seen inside the cabin before takeoff?
I saw this quite thick white smoke like thing in the cabin in my last flight, before takeoff that is, while it was taxiing and loading passengers:

Video
Please watch in HD to best notice the phenomenon.
It seems to be coming from the AC vents of aircraft. I think it's a quite common phenomenon as the cabin crew didn't even pay any attention to this.
But I saw it for the first time. What is this?
Aircraft Details : VT-IDE (A 3.5 year old A320)
The departure location was Bhubaneshwar and the approximate weather was:
Temperature : 28 °C
Humidity ~ 85%
Clear Sky, good visibility

See this video for an explanation of that vapor: https://www.youtube.com/watch?v=om8Xwa3MhaU

Oh no! The ground crew accidentally connected chemtrail feed line to the ac connector

Short explanation
You're basically seeing this:

In this picture fog appears because air is humid and the ground is colder than air. In your case humidity from the air has condensed into water droplets because of a temperature drop in the cabin, near the AC vents.
Air is a mix of invisible gases, of which water vapor. Water is air-soluble, but only to a certain extent for a given temperature. Humidity is measured by the relative humidity index, the ratio between the mass of water vapor dissolved and a mass of dry air: 0% for dry air, 100% for saturated air, i.e. containing the largest quantity possible given the temperature.
If drying time is considerably reduced in summer, this is because hot air can absorb much more vapor than cold air, as depicted on this psychrometric chart calculated for sea level and 100% RH:

(Source)
Read this curve as: For temperatures -18°C, 20°C and 28°C, one kg of dry air can be saturated (100% RH) with respectively 2g, 17g and 27g of water vapor. When the mass of water vapor exceeds this ratio, water vapor in excess condensates into liquid droplets which separate from gaseous air and form with it an aerosol.
This condensation can occur after two events:

A large quantity of water vapor is introduced in air, e.g. by boiling water or by evaporating water.
Air temperature is lowered, mechanically increasing the RH, this is a way to get water in dry deserts.

From left to right: Boiling water, evaporation tower, liquid nitrogen. All cause water in air to condense after reaching 100% RH. This same mechanism is able to create mist, fog, clouds and dew.
The other parameter that can decrease water capacity and induce condensation is a significant change in pressure. In aviation this can be seen in case of explosive decompression, fog fills the whole aircraft in 1/10s, and more casually in the low pressure areas of the wing:

(Source: JetPhotos, by Seth Jaworski)

Calculation for your case
In your case:

Ambient air temperature is 28°C, it means 100% RH corresponds to 27g of water vapor in one kg of air, and 23g/kg for a RH of 85%.
The temperature for which 23g/kg represents 100% RH is 27°C (the dew point).
The spread between current temperature and dew point is only 1°C, condensation  is going to happen easily.
Around the AC vents, air is cooled at 20°C, this is far below the dew point. For this temperature there are 6g/kg of gaseous water vapor in excess, they are condensed into 6g of visible water droplets. This happens only around the AC vents, remote from the vents the temperature is warm again, water droplets are evaporated accordingly.
Water (either vapor or liquid) is progressively eliminated from the cabin by the AC system, RH is maintained around 20%, condensation cannot occur anymore.

Condensation appears near the AC inlets:

Air circulation in large pressurized aircraft, from this answer

Dew point
Psychrometrics, the branch of physics studying how liquids and solids boil or condense, revolves around the notion of equilibrium vapor pressure, but for us mere mortals, the notion of dew point is more practical.
The quantity of vapor air can contain before being saturated is called its water vapor capacity, it increases exponentially with temperature. Let's define two cases:

Full saturation: If the quantity of vapor is the maximum allowed for the current temperature (100% relative humidity), decreasing the temperature or adding vapor will cause some vapor to condensate into water.

Partial saturation: If air is not fully saturated, there is a (lower) temperature for which it becomes saturated. This temperature is referred to as dew point. So whatever the current temperature, fog can be created by decreasing the temperature until the dew point is reached.

For sake of accuracy: When water vapor is present in air, it is continuously condensed and evaporated. The dew point is the point where condensation occurs at a greater rate than evaporation and water droplets start accumulating.
Whether fog will form depends on the the dew point spread, that is the difference between current temperature and dew point. When the spread is large, the change in humidity or in temperature must be large. In your case, the spread is small, because air is wet and temperature is high.
Measuring the dew point
The dew point may be determined with a psychrometer (a system with dry and wet thermometers):

Whirling (how to use it) and digital psychrometers
Pressure, dry and wet temperatures, dew point and relative humidity are linked by the mean of vapor pressure. By knowing three elements, the two other can be determined. So knowing pressure, dry temperature and wet temperature allows to determine the dew point. It exists precalculated charts (Mollier diagrams) to read dew point directly. There are also online calculators.
Dew point and fog/mist information
KLAX ASOS report:

As previously mentioned, when the dew point is close to the ambient temperature and the humidity percentage is high, the visibility is low and fog occurs.
When water vapor changes into ice
When the dew point is below 0°C, water vapor is not condensing into water droplets, it is converted directly into ice crystals (frost).
This effect is particularly dangerous and can lead to icing in the carburetor of a piston engine. This condition can happen in areas of low pressure and low temperature, commonly created by the sudden pressure drop in the throttle venturi, due to venturi effect and the temperature drop due to fuel vaporization.
For a typical piston engine, this effect is centered on a external temperature around 10°C but extends to a large range of temperature when the engine is slowed down (descent):

Induction icing conditions
The remedy to carburetor induction icing is to heat the carburetor and raise the due point to a safe area, especially when descending.
For jet engines, induction icing (not to be confused with flying in atmosphere with ice crystals or supercooled water droplets) can cause ice to form at the engine inlet. Blocks of ice can then break, causing damage to blades, compressor stall and/or combustor flameout. It can also cause EPR probe (PT2) reading errors and subsequent FADEC fatal behavior.

But don't the packs specifically extract humidity from the air conditioning air? So how does it happen that the cabin is humid? It is normally kept at around 20% or so.

Please see the update about weather..

Super-short answer: it's a cloud forming inside your plane.

I was once informed by the Weather Channel that I could expect 103% humidity on an absolutely miserable June day in New York City. I was never sure if that was some kind of error in their prediction algorithms, or if it was possible to achieve super-saturation in a whole city like that.

@JanHudec this might be something that usually happens when an AC system just fires up, at least when I have seen it in a car, but planes like these don't have evaporator/condenser systems like automobiles do.  They operate by compressing and re-expanding the air causing it to cool rather than passing uncompressed air through a Freon-compressing heat-exchanger.  The hotter compressed air may absorb more moisture and then when it expands again, now cooler, the water in it condenses and becomes visible.

@mins you're right that makes sense as well.  The colder air coming out cools the warm moist air around it and causes condensation.  Am I correct that transport aircraft don't use systems similar to those found in cars?  Ie. no freon with condenser and evaporator heat exchangers?  I'm curious because the company I work for manufactures heat exchangers for Nissan.

@RyanMortensen, the air is only hot in the bleed air ducts and there is nowhere to pick moisture from there. Quite the opposite; the aircraft systems specifically remove moisture from the air. The only case where this happens is when the cabin is filled with moist ambient air and the packs just start feeding cooler, though also drier, air.

@JanHudec ten-four!

Is there a simpler worded explanation, one which I can tell to my co-passenger (if its a girl), if this kind of situation happens in my flight, in the hope of impressing her..

Then how come absolutely none of this "water vapor" actually condenses into water droplets on the cabin baggage doors, seats, ceiling, or floor? The interior of the plane would be awfully musty smelling after a few of these cycles. The "water vapor" doesn't inherently feel moist when one holds a hand up to it, and one's hand does not get wet.

@GooseMcGoose Unfortunately "absolutely none of this "water vapor" actually condenses into water droplets on the cabin baggage doors, seats, ceiling, or floor" isn't true on many aircraft. I've had condensation from the AC vents drip water on me while taxiing out in very warm, humid weather several times. Some designs are worse about this than others.

@Firee See David's super-short answer above. If you require one even shorter than that, I suggest, "It's water." :)

I would imagine that the problem is minimized (but not eliminated) at airports where pax are loaded via jet-bridge. The seal between the jet-bridge & aircraft is by no means water-tight, but it's not a large hole into which massive amounts of ambient air will rush. I'd think that the majority of the air entering the plane via the pax door would be conditioned air following the pax down the walkway from the terminal. Of course, this ignores the large holes in the fuselage where luggage is entering...

This isn't an answer, it's an education!

The bigger question to me is why I’ve only seen this on A320s.  I wonder how their AC differs from other planes I’ve been on?

@KRyan Yes it's possible. We would see it all the time on the central California coast. It would happen when you're on the coast with winds coming off the water. With a marine layer (fog), the air temp and dew point will be close to the water temp (100% RH). When the wind blows the air mass over land, and the air temps over land are cooler than the water, you can get that super-saturated condition. Generally, an RH of 101% - 103% was the highest we would see.

It's condensed water droplets. Outside air with high humidity is reduced in temperature from 28 - 20 ºC (after going through a compression - cooling - expansion cycle), and some of the water vapour condenses since colder air cannot contain as much water vapour as warmer air. The condenser is located before the expansion turbine. Indeed like @David Richerby says in a comment: it's a cloud forming inside your plane.
A home air conditioner has a compression - cooling - expansion cycle as well, yet this white cloud never appears at home. Home air conditioners cool the air to a temperature lower than final temperature, condense the water out, then heat to the final temperature so that outflow relative humidity < 100%. Aircraft A/C systems are not dimensioned for this, since the phenomenon only occurs in some airports, during ground handling and taxi. As soon as the outside air temperature is less than 20ºC the clouds cannot appear within the aircraft.

EDIT
For a brief moment an answer that was a question and should have been a comment appeared: why do the droplets not condense, and why don't they feel wet when you put your hand in. Because there is not enough time for all of that: the water is blown into air that is warmer and has a lower humidity than 100%, and dissolves again. Water condenses on a surface when the dissolved water vapour meets a surface that is colder, cools the air locally, and provides opportunity for the water molecules to find each other and congregate into condensation drops.

The above explanation is only a technical explanation of how mist forms.
The reason you see it on the A320 in those conditions is this.
Every AC system (home, building, car) uses a plant to generate a cold refrigerant/water, which in turn cools a heat-exchanger. The supply air is passed over this heat exchanger, and the humidity condenses on it and drops off. You never see the mist becasue what is discharged is dry, cold air.
Aircraft AC is different. A plant is too heavy, and AC is rarely required on an aircraft anyway since they spend most of their time in an environment which is -60C. What they do require is pressurisation.
Therefore, the pressurisation and AC are a combined system - and it works on the supply (ambient/outside) air. There is no refrigerant. The air is compressed in the engines or APU, that air is extraordinarily hot. It is cooled (it only needs to be cooled a bit - maybe 50C or so) and then brought back to ambient pressure. When brought back to ambient pressure - even ambient air of 50C is now below freezing. This air is discharged into the cabin. This can't hold the same moisture as outside air, so mist forms. You'll notice that nowhere in the cycle is the moisture able to condense onto a heat exchanger. Some aircraft use dessicant wheels, but these are not as effective as condensing moisture onto a heat exchanger.
After the aircraft takes off, the humidity is lower, the temp is colder and the phenomena is no longer visible.

The air is dry in every case, home AC, car, airplane, etc, because it was cooled. The method of cooling makes no difference. Your explanation is good, right up until "so mist forms," at which point you've lost me.  Mist forms because the air from the pack is cold, not because it's any different from any other parcel of air otherwise. Cold air + humid air = mist.  Refrigerant or not plays no role in the formation of mist.

"When brought back to ambient pressure - even ambient air of 50C is now below freezing."  What are you trying to say here?  Downvoted for this confusing statement, and for "me-too" answer to a 4 year old question.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How is a fuselage 'puncture' repaired?

Operator on phone, GoAir plane hits aerobridge

How is such a 'puncture' repaired? Is the cabin pressurization also affected by such a puncture, and approximately what is the cost incurred to repair such a puncture?

"Operator on phone ...  hits aerobridge" ... say what? Pilots are texting too? Smartphones are THE BANE OF HUMANITY and the ROOT OF ALL EVIL... :p

While I don't entirely disagree, @CGCampbell, it was the air bridge operator yapping on the phone, not the pilot. Just remember folks, this is what an air bridge, moving a 1-2MPH can do, imagine what your SUV moving at 60+MPH will do! /soapbox

BTW for those in the U.S., what OP is calling an "aerobridge" is more often referred to stateside as a "jetway" or "boarding tunnel".

The first step for repairing damage to a fuselage skin panel is to cut out the damaged area1. Cracks tend to propagate, so an area much larger than the damage will be removed to ensure that all damaged material is removed. The stiffeners that are behind the fasteners will need to be inspected for damage as well.
A filler plate will be cut to fill the hole in the skin. One or more doubler plates larger than the removed area will then be cut and all plates will be riveted together. The doublers will possibly be on both sides or at least thicker than the surrounding skin. See here for some examples.
It's likely that a larger repair such as this one will need to be inspected regularly to ensure that the repair holds strength.
It's hard to tell where exactly this damage is located, most of the aircraft skin is part of the pressure vessel. However, the airplane is not completely airtight, so a certain amount of leakage is normal. A hole of this size will probably not prevent the fuselage from pressurizing, though larger ones will.
The cost will depend on the extent of the repair, but could be from \$5,000-\$15,000 (300,000-1,000,000 INR) based on this estimate.
1Actually, the first step in repairing damage to an airplane is consulting the structural repair manual (SRM). This is provided by the manufacturer and describes general methods for repairing minor and typical damage to airplane structure. If the damage is beyond the allowances in the SRM, the manufacturer will be consulted. All of this is to ensure that the repair will meet or exceed the strength and life of the original structure, or if this is not feasible, that the limitations of the repair are documented and understood. Major repairs will require extra inspections to ensure that the structure is still in good shape. Aircraft skin will not be welded, since this can cause heat damage to the metal, requires careful skill and care to do properly, and will not hold up well to fatigue.

Chapter 4 of the AMT has some really good illustrations of the various patch techniques around page 4-95 or so :)

It would also be nice to know how much time such repairs typically take, to figure in the cost of not having the A/C in use. Perhaps lost revenue could be more than the actual cost of repair?

@ALANWARD could be anywhere from a few hours to a few days, depending on a bunch of factors: how large the damaged area is, whether it's in a structurally important area (may require a more involved repair), and whether the mechanic working on the damage has easy access to both sides of the hole spring immediately to mind. I'm pretty sure the lost revenue from being out of service is significantly higher than the repair cost, but compared to the cost of a new skin panel (or a new plane) repairs often make good financial sense.

@voretaq7 OK, that's about what I was thinking myself. Thanks for commenting.

For commercial airliners and most pressurized craft, fooot's answer is good. For most light planes, a repair of this type usually boils down to replacing the entire piece of formed sheet metal or fiberglass that was breeched.
An old acquaintance of mine owns a Cessna 150 that she bought from a farmer who'd lost his medical; the plane had been sitting in a field since then until some rednecks came by and took a potshot at the tail with a hunting rifle. She got the plane for a song and also lucked out on a hangar nearby, but the plane couldn't be certified until the dorsal strake that had taken the hit was replaced. From what I gathered it was a fairly simple matter of removing the old pop rivets with an angle grinder and nailsetter, lifting off the old strake, fitting the new one and pop-riveting it back into place, then paint to match. That particular piece was probably the best-case scenario to have to completely replace; most of the rest of the tail is in large sheets and the bullet would have punched clean through two of them requiring a more extensive teardown to remove and replace.

Whether the panel gets replaced on light aircraft depends on a few factors - plenty of planes have skin patches made the same way fooot described - if it's a small area of damage on a relatively large panel it's easier & cheaper to cut out the damage & install a patch (flush or scab) with a doubler than drill out all the rivets to replace the skin.

The outer fuselage is not the pressure hull. The pressurized cabins are a different shell and are inside the fuselage. This kind of damage would have no effect on cabin pressure. Even if the cabin hull had been rent by a hole of this size, the effect would be minor. The pressure hull is actually open to the air anyway. Pumps continuously add air to the pressurized area while air flows out through various points. The hole would have to be much larger to defeat the power of the pumps.
To make the repair two things need to be done: the larger open hole needs to be patched and the dent and abrasions around the framing member need to be corrected.
To repair the larger tear, the sheet metal worker will probably cut it out in a rectangular pattern with rounded corners. Then a backing plate will be fitted to the rear of the panel and welded or brazed or riveted in place (depending on the materials involved). Finally a patch will be made that fits into the rectangular recess exactly. This patch will be welded and/or rivetted to the backing plate. The patch will then be sanded smooth and painted. You will barely be able to tell it is there when it is done.
To repair the dent, it will first be pulled out by welding a stud onto the dented portion, then the stud will be pulled outwards. This is repeated until the dent has been removed. Then the small abrasion on the dented area and in the vicinity of the framing member will be filled in using a welding machine. The weld will be abrasively smoothed over, then everything repainted.
The cost to do the repair will depend greatly on where and who does it. In the Boston area I would expect it to cost about $3,500.
If you are interested in the details of acceptable methods of repair, the FAA provides a manual describing typical methodologies: Aircraft Inspection, Repair & Alterations: Acceptable Methods, Techniques & Practices (FAA AC 43.13-1B and 43.13-2B) .

Sheet metal work, brazing, welding and dent pullers? This might work on GA but is certainly not the repair process for a highly stressed modern fuselage.  Take a look at this for an idea of the complexity.  I think Airbus might want a bit more than $3.5k.

The outer fuselage is not the pressure hull. For most of the fuselage this is not true.

@Simon The article you link shows the same method I describe. The piece described as the "repair doubler" is the same as the "backing plate" in my description. I am simply using a different term. I doubt the repair will be done by Airbus. There are specialist airframe sheet metal workers that do this kind of work.

@TylerDurden you keep talking about welding though. On the fuselage this is not going to happen.

@foot There are only two main ways to fix a tear like the one shown: the way I describe and a bonded repair. A bonded repair is much more expensive and I doubt they will do it in this situation. Both brazing and welding are typical and FAA-accepted methods of fastening in skin repairs.

@TylerDurden Referencing the link you added, only when there are no manufacturer repair or maintenance instructions. This data generally pertains to minor repairs. And: 4-89. AIRCRAFT PARTS  NOT TO BE WELDED...    d.  Nos. 2024 and 7075 Aluminum. I searched a fuselage SRM and welding only had one limited use.

@foot That is a general guideline, however, 7075 is indeed welded in certain repair situations such as this one. A special kind of welding machine, called gas tungsten arc welder (GTAW), is used which limits the heat dispersion. Certain operational specifications and parameters have to be obeyed. If you talk to an aircraft sheet metal worker, they will verify this.

Let us continue this discussion in chat.

For a part 25 airplane you will make the repair according to the Structural Repair Manual (SRM) or an approved engineering order. The info in part 43 are acceptable methods to repair an aircraft, but when you're talking about part 121 operations and part 25 aircraft you're in an entirely different game.

"The outer fuselage is not the pressure hull. The pressurized cabins are a different shell and are inside the fuselage." Where in the world did you learn this? We're not talking about ocean-going oil tankers with double hulls.

The pressure hull to save weight is the skin of the aircraft in most cases. This would exclude the skin that extends to the wings and tail sections. This also helps mitigate issues related to expansion and thermal transfer if there was a skin and structural members affecting an internal pressure hull.

@SportsRacer: Maybe he confused it with a submarine?


text:
Was this a successful deployment of an MD-88 tail cone evacuation slide?
In this video, the tail cone is popped off as part of the evacuation.
It stays attached on a lanyard which seems to be more of a hazard than letting it fall free.  Additionally, the slide doesn't deploy.
It's clear that the rear and front port exits are the safe ones, but did something go wrong here?
(P.S.  I'm not talking about the actions of the pax, just about the tail slide).

This shows a succesful tailslide deployment of a related type: https://youtu.be/TAndXzhgurI?t=75

I know you aren't talking about the actions of the pax, but its amazing how many people decided to grab their carry-on when evacuating...

"Additionally, the slide doesn't deploy." Doesn't that observation comprehensively answer the question, "Was this a successful deployment of an MD-88 tail cone evacuation slide?"

@DavidRicherby No.  For example, is there a mechanism whereby the port slides deploy but if there fire is towards the rear, the tail slide does not but the cone is popped anyway - perhaps to vent the cabin?  BTW, that was not my title.  The Aviation.SE fetish to edit every title into a question itself continues and I can't be bothered to undo it.  My question was simply "did something go wrong" since I don't know if the slide is supposed to deploy or not.

@ROIMaison They totally cheated in that demonstration. I hope in a real evacuation there's a tug sitting there with a pallet to pull the tail cone out of the way of the slide

@Simon - Oops!  Go ahead and change it back.  My apologies.

@ronbeyer sadly it seems people never change

Considering that the slide was not deployed, it is definitely unsuccessful. The MD-80 series (also the DC-9 and boeing 717) had tail cone assemblies which could be jettisioned in case of an emergency.
When operated, the tailcone is expected to swing to the left of the fuselage due to to its own weight because of the way it was rigged.

The jettison cone is attached to the slide through a lanyard, which passes through two eyelets in the tail cone and is attached to a spring in the middle of the cone.
As the jettison mechanism is activated, the tail cone is released and because of the way it is rigged, turns 90$^\circ$ and moves to the left of the aircraft to keep it from interfering with the slide deployment.
As it falls, it pulls on the slide container via the slide lanyard, moving it so that it can inflate and delpoy, while the tail cone remains to one side of the aircraft.

You can see it in the images below:

All images taken from Accident Report IN-045/2004
The following image shows the undeployed slide (in an MD-81) from the inside along with the lanyard.

Image from aircrewbuzz.blogspot.in
The release can be made by three ways- there is an emergency handle on the aft bulkhead door, one at the end of the catwalk and another one in the exterior of the handle.
In this case, the tail cone has pulled the lanyard, but the slide has failed to delpoy. While I'm not sure why the cone is hanging, it appears that the lanyard has snagged somwhere, probably in the cam rotator.
Note: This would not be the first time the slide has failed to deploy/deployed unsuccesfully or has deployed inadvertently. Also, see this discussion on the MD-88 tailcone jettison and slide deployment mechanism here.


text:
Where do contrails that grow wider get their condensation nuclei from?
When a contrail occurs in a pure and over-saturated air in high altitude, it's also because of the engines emit condensation nuclei.
But if the contrail expands in a wider cirrus, where do the contrails gets the condensation nuclei from? (I'm guessing it's necessary for the expansion of the condensation).
Or can the cloud grow without additional nuclei ?

Supersaturated environments just need the initial condensation in order to start the entire condensation process. It doesn't need to find more condensation nuclei particles. The condensation nuclei generated by the engines is enough to start the chain reaction for a cloud to form in the supersaturated environment.
Take this video as an example. It is an aqueous solution but the situation is the same: a single crystal causes the rest of the solids to precipitate out of the solution.

Thank you for answering. Do you know what I could search to understand the phenomenon better please? I admit I don't know what keyword to search. And "Condensation chain reaction" gave me poor and unrelated results so far.

@vmonteco: There is an article in Nature, Formation and radiative forcing of contrail cirrus, which explains how the cirrus expands from the contrail.

@mins Thank you very much !


text:
What are the differences in aircraft performance between tractor and pusher propeller designs?
Some aircraft have pusher type propellers.  In what ways is it different from a tractor type propeller?  What's are the advantages and disadvantages over a tractor type.

Do you mean actual differences in the propeller itself, or differences in aircraft performance between tractor and pusher designs?

performance between tractor and pusher designs

See this answer to a similar question.

Well one major difference is the blade pitch, pusher props will seem to be backwards.
You must also modify the engine's bearings to be able to support the propeller mounting and crankshaft in compression, rather than tension.
A pusher prop's engine also has to be cooled differently than a tractor, because the propeller is not blowing air over the engine. Therefore some ducting and ram air magic has to be done, or some other system of cooling implemented. Many pusher-type planes have had severe issues with cooling.
Note: some of the next few might not necessarily apply to pusher-type planes where the engine is mounted above the fuselage, like the Lake Amphibian and many other seaplanes, or some other oddities like the B-36. I am talking about more conventional pushers like the Velocity, Beech Starship, or Cessna 337.
Another issue is FOD. A pusher prop will likely be mounted behind at least one of the wheels, which can kick dirt and rocks up into the propeller. Now imagine a prop that is very close to one of the mains. Even on paved runways you encounter rocks and debris occasionally, and it only takes one hit in the right place to produce a failure-causing nick.
Compounding on that last one, when a tractor-type plane takes off, beacause the prop is forward of the center of mass, the propeller will rotate up, away from the ground. But a pusher prop is usually just the opposite: it will rotate down a bit, increasing the chances of a prop-strike. This is one that is heavily considered and worked-around while building pusher-type planes, but while you can mitigate the danger by placing the prop closer to the CM, or making the landing gear taller, you can't completely remove it as far as I know.

Performance-wise, a pusher is theoretically better. This is because the high velocity propwash isn't scrubbing over the fuselage or wing, introducing more drag (or "negative thrust" if you like).
Conversely, a tractor's propwash is always creating extra drag over downstream surfaces - more drag than if those surfaces simply were scrubbed by the plane's airspeed.

the stream tube that goes through the propeller disc is still moving faster than freestream when it goes over the wing, causing more drag. There isn't (or at least I wouldn't expect) any vortical component to that flow, however, like there would be in a tractor configuration.


text:
Does a multi-engine rating propagate down from commercial to private?
I have a private pilot license with airplane single engine land and instrument ratings. I'm about to get the commercial with a airplane multi engine land class rating.
Let's say I'm not currently flying for hire and my second class medical is more than a year old, so I cannot exercise my commercial privileges. Can I then legally fly a multi-engine plane exercising my private privileges, carrying, say, a friend or other non-paying passengers in a part-91 operation?
In other words, does the class-rating propagate down to my private?

please consider marking the answer correct. thanks

I already accepted an answer as correct, and that answer has since disappeared (apparently deleted by author). I'm marking your answer as correct now.

Note Part 61.31(c):

(c) Aircraft category, class, and type ratings: Limitations on the
  carriage of persons, or operating for compensation or hire. Unless a
  person holds a category, class, and type rating (if a class and type
  rating is required) that applies to the aircraft, that person may not
  act as pilot in command of an aircraft that is carrying another
  person, or is operated for compensation or hire. That person also may
  not act as pilot in command of that aircraft for compensation or hire.

Since you will have the Category and Class rating, then you may carry another person.
Your old license says:
Private Pilot
Airplane Single-engine Land
Instrument Airplane

Your new license will say:
Commercial Pilot
Airplane Multi-engine Land
Instrument Airplane

Private Privileges
Airplane Single-engine Land

You may fly under part 91 ASEL, AMEL, and IFR
You may fly part 119 or 135 (or other commercial operations) AMEL and IFR1. You may NOT fly part 119 or 135 or any commercial operations ASEL. If you want to fly ASEL in a commercial operation, you will need to upgrade your ASEL by passing another commercial checkride in an ASEL.
This is the back of my certificate. I took the ASES checkride when I had a Private certificate.

1. Assuming you have the correct medical and any other checkouts you may need

Stated differently, a commercial certificate also conveys privileges of a private pilot certificate.


text:
What fail safe protections exist to prevent non-plug type cabin doors opening in flight?
Increasingly, modern types do not use plug doors to save complexity and weight.
As far as I know, the doors automatic/manual interlock only controls whether the slide inflates when the door is opened.  The doors have electrically powered locks and EICAS alerts to show unlocked doors.
However, anything electrical can and does fail.  What fail safe protections are in place to prevent non-plug doors from being opened in flight?

From what I can tell, the cabin door, e.g. in Airbus, don't have any electricity in them. They have big lever that pushes several massive latches to the frame on the sides. And the interlock can only be closed when the latches are in their end position.

@JanHudec So what stops someone pulling the big lever?

I believe when there is a pressure difference, the friction on the latches is more than human can overcome. I don't have any hard reference for this though.

Note that the cargo door do have electric mechanism and on 747 there was an accident where the locking mechanism got energized in flight (via short) and did have enough power to break the interlock, overcome the friction and opened the door (the plane was severely damaged, but managed to land). But humans don't have as much strength.

Which types are using non-plug doors for other than the fore and aft cargo doors? (Ignore main deck cargo doors on freighters for now)

sounds like a whole new question, @UnrecognizedFallingObject

@UnrecognizedFallingObject Yeh, new question.  Most Airbus types for one.

@Simon it'd be helpful if you listed which types your believe do not have plug doors or which type you are interesting in.

Downvoter.  Care to explain why?  I don't care about the points, but I do care about understanding what's wrong with the question.  Thanks.

I am not sure about all aircraft types, but on the 777 cabin pressure is still used to positively lock the door into position in flight. When the door is open, the door is in a slightly raised position. As the door is closed, rollers on the door are lowered into tracks on the door frame. The tracks are curved slightly outwards, so to open the door the it most be moved slightly inward, which is impossible even with a slight amount of cabin pressure. The actual pressure load on the door is carried through the stops, which are inboard on the door and outboard on the frame. Once loaded with the pressure, these stops would also have a tremendous amount of friction preventing the door from sliding upward. Note the vent door near the top of the door in the picture below, this door is attached to the latching mechanism and it opens in the first movement of the handle, again preventing the door from unlatching if the cabin is pressurized.
Refer to this picture of the 777 door, to see the rollers, tracks and stops (Source).

I believe the 777 design is very similar to the Airbus design. There is a very good video showing the operation of Airbus doors on YouTube, here. It shows how the door must be lifted above the stops, before it can open outward. If you watch closely you can see the move slight inboard before lifting. This is a quote from the A340 maintenance manual: The lifting arms of the lifting shaft (which support the passenger door in roller fittings attached to the door frame) move the passenger door a small distance into the cabin and then upwards."
Note, some people (including Airbus and Boeing) still consider these doors plug designs since pressure still loads the door against the stops. For example, Boeing specifically states the 777 doors are plug type doors.

The main reason for my curiosity is that Airbus doors for example do not move inwards at all when opening and are not larger, in any dimension, than the door aperture so surely cannot be classified as plug doors? I think the answer to my question is that since the pressure applies an outward force to the door, the locking mechanism has a great deal of friction which it would be impossible to overcome regardless of interlocks.

@Simon they are very similar to the 777 door in that it slides down into its stops so pressure loads it against the stops. I've got some data at home I'll post later, but Airbus itself calls them plug doors. Also if you watch the video very closely you can see a slight inboard movement, just like a 777.

@Simon Sorry for the long delay, this is from the A340 manual: "The lifting arms of the lifting shaft (which support the passenger door in roller fittings attached to the door frame) move the passenger door a small distance into the cabin and then upwards."

Thank you!  I'm now wondering if these are still correctly described as plug doors?

@Simon I think that is probably a little bit of a philosophical question. My take is that Boeing and Airbus call them plug doors, and they are definitely door experts. I think the term fits since the stops on the door are wider than the stops on the frame, so the door is technically bigger than the frame, just not everywhere.

"again preventing the door from unlatching if the cabin is pressurized" - and, conversely, preventing the cabin from pressurising if the door isn't securely latched.


text:
What are the rules for "see and be seen" when you suspect a potential conflict in class G airspace?
The airfield I learned to fly helicopters at is in uncontrolled airspace and has an air/ground radio service only.  It has a main runway, 07/25 and a secondary runway only used in high x-winds, 13/31.

All fixed wing circuits are to the North so fixed wings will fly right hand circuits when operating Westerly and left hand circuits when operating on the Easterlies.  Helicopter circuits are always to the South so the reverse is true.  This works well and keeps fixed wing traffic separated from helicopters.
Therefore, there is no "dead side" since the Southern leg is the downwind for rotary and the Northern leg is downwind for fixed wing.
Once, when preparing for take off from runway 25, I heard a call from a fixed wing announcing they were about 7 miles to the East and would "join dead side".  The A/G operator reminded the pilot, who clearly wasn't one for reading the relevant publications, that there is no dead side and advised them to make a standard overhead join.  We were operating to the West so my circuit would be left hand to the South.
The next call I heard from them was on the downwind leg when the pilot announced "descending dead side".  The operator reminded them that there was no dead side and more insistently instructed an overhead join.
I am now at 1,000 feet on my downwind with an aircraft I suspect is in front of me heading towards me and descending!  Referring to the plate, I am heading roughly East to the South of the road marked "A303" at the bottom of the plate.
I took 3 actions;  I made a position call of "downwind South", increased my height to 1,200 (I figured they were likely to have joined at 1,000 feet) and switched on the landing light.
I can't find any rules of the air or rules of thumb to help in this situation.  Are there any?  Is there anything else I should have done to help to avoid a conflict?
I never did see the other aircraft and I knew from the pilots actions and their lousy use of the radio that they were inexperienced or incompetent and I didn't go looking for them when I was back on the ground to "share a little wisdom" with them.

I think a few more details would help make this question clearer. Which country and regulations are you talking about, I guess the UK? What were the actual runway numbers (so we can visualize what happened)?

@Pondlife Thanks. I edited in the runway numbers and also added the caa-uk-regulations tag but since this is class G, VFR only and an uncontrolled airfield, I doubt that the formal regulations cover it.

There are no rules for what to do when others don't play by the rules. It all comes down to 'good airmanship' and you seem to have it under your belt.

In the USA, RW aircraft are supposed to avoid the flow of FW traffic and FW traffic have the right of way for landing.
So I would have descended to 500ft instead of climbing. Slight differences in altimeters make 1,200 and 1,000ft too close for comfort. But  500ft AGL (noise abatement be damned in a safety situation) is a perfectly acceptable altitude in a heli.
Also, I would have adjusted my downwind so that I was fairly close to the runway where airplanes are rarely found.
So now I have both vertical and horizontal separation, and I am in a continuous position to land, at my convenience.

Thanks for this, and adding your thoughts into mine, I believe that this answers the question. The first comment is not applicable since there should have been no FW traffic where I was.  However, descending would have been better.  My thinking at the time (all done in the space of a few seconds!) was that the FW would be descending and I would therefore climb but that is probably why I didn't see them.  As we all know, spotting traffic above is easier than below.  I could also have adjusted my horizontal spacing and moving towards the runway makes complete sense.

"there should have been no FW traffic" the right-of-way rules (in the US, at least) don't have anything to say about who is supposed to be where, only what you're supposed to do if there is a conflict.

My point is that under UK regulations,CAP 032, the AIP, explicitly states that there is no deadside and that FW traffic should use an overhead join.  Should there be a reportable incident, which there would have been if there had been an airprox, the CAP is enough to sustain a prosecution.  In that respect, the FW should not have been there.  I was not "in the flow" of FW traffic.

@Simon different countries different regs!

Of course, which is why I tagged it as UK CAA.

Of course, which is why I wrote "In the USA..."


text:
How is an airline call sign assigned?
Commercial aircraft are identified on the radio with an airline call sign and their flight number.
The answer on this related question mentions:

the telephony designator of the aircraft operating agency

How is this designator assigned? Are there requirements for getting one?

These designators are assigned, on request, by ICAO. The request is made by the applicant through their National Aviation Autority.
There are actually two designators: the ICAO three letter code and the ICAO R/T designator.
All assigned designators are published in ICAO document 8585.
Two things are to be considered:

The three letter code must be unique to the aircraft operator
The R/T designator (callsign) must not cause confusion with other operators flying in the same area,  preferably at most three syllables long and it must be pronounceable in at least one of the following languages; English, French, Spanish or Russian.

Since there are many operators that have "airline" or "airways" in their name, that word is usually avoided in the R/T designator as it easily causes confusion.
For pilots it is not too difficult, they are typically paying attention specifically to their own R/T callsign. For controllers it is more difficult since they have to associate the three letter code on their screen with R/T designators that or not related in an obvious way.
For example:

British Airways has three letter code BAW and R/T designator
"SPEEDBIRD".
Thomas Cook Airlines Ltd has three letter code TCX, and R/T
designator "KESTREL".
Thomas Cook Airlines Belgium has three letter code TCW, and R/T
designator "THOMAS COOK".
Thomson Airways has three letter code TOM, and R/T designator "TOMSON"

Since the application goes through the NAA, there are slight differences in the procedure from country to country. Some examples are listed below:

Netherlands

AIC

USA

FAA Advisory Circular AC 120-26K and
FAA Order JO 7340.2D.
NBAA call-sign application guidance website


text:
When should I use the global Reynolds number and when the local Reynolds number?
I know that for the wing, the Reynolds number is based on the MAC. This makes sense to me, as the boundary layer develops along the length of the wing.
However, what kind of Reynolds number needs to be used if there is a flap behind the main wing and there is flow pattern like this:

Source
Some of the air particles  will have streamed along the wing before hitting the flap, so I would say then you should use the wing Reynolds number.
But, on the flap, also a new boundary layer will start. Particles coming through the gap will only see the flap, and will thus experience a flow with a lower Reynolds number (based on the flap length).
If I'm talking about flow phenomena on the flap, should I be using the global Reynolds number (based on the wing chord), or the local Reynolds number?

What are you going to use that Reynolds number FOR?

This is not the exactly the usecase that I'm working on, but I've got the velocity field of a vortex behind a big body, and somewhere in this velocity field I want to place a fiarly small object (I think small enought to say it will not distort the velocity field I have). For this object I have some windtunnel data at different Reynolds numbers, and I was wondering what Reynolds number I should use.

x @ROI: Your windtunnel data should come with a specification of where on the object the length measurement that their Reynolds numbers are computed from applies.

I know what the Reynolds number of my windtunnel data is based on. I was just wondering if I should take the influence of the big body into account in selecting the Reynolds number. But I guess this is not the case.

Definitely use the local Reynolds number on each of the flaps! The flap will only work properly if it is allowed to produce its own, fresh boundary layer. If instead it inherits the slow, thick boundary layer of the wing or flap ahead of it, the flow would soon separate and the flap would loose most of its effectiveness.
Note that a Reynolds number is always a local phenomenon. At 2 m from the leading edge, the Reynolds number is twice as large as at 1 m. Only when you compare total coefficients for the whole airfoil would the Reynolds number at the trailing edge be used.
If the distance between your bodies is more than a few centimeters, the boundary layers will not interact, so again the local length should be used for the Reynolds number reference length.


text:
How can true altitude be calculated from pressure altitude, temperature and altimeter setting?
The manual that came with my Jeppensen E6B has the following sample...

If an aircraft is flying at 12,500 feet with an outside air
  temperature of -20C and the altimeter is set on 30.42 inches of
  mercury, what is the true altitude?

The explanation goes on to find a pressure altitude of 12,000 feet.  After placing -20C over 12,000 we find 12,500 (12.5) on the B scale and read the true altitude of 12,000 (12.0) on the outer ring.
Looking closely at the outer ring, I see that the true altitude is actually just slightly below 12,000.  Now for practical purposes I realize we don't worry about differences like this and round the result.  But I'm working on a project in which I need to calculate the true altitude precisely.  I'm having a difficult time finding the formula.  The closest thing I have found says the correction is 4 feet per thousand feet indicated per degree off of ISA.
When I attempt to apply that formula to our sample problem I get 4 * 12.5 * -35 = -1750.   Applying that correction gives a value far below expected so obviously I'm doing something wrong.  Can someone straighten me out here?  References to documentation with educational value are especially welcome.

Can you add more detail and context?  In practice, it is impossible to calculate true altitude from pressure since the atmosphere does not act in a linear way.  One day A, with ISA standard at sea level, the pressure altitude for say 900Hpa can be different for day B with ISA standard day.  It will also change from minute to minute.  So when you say you must calculate it exactly, we need to find a different approach.

If you need true altitude, you should not use pressure altitude, especially at higher altitudes. Use GPS instead. Out of curiosity what do you need true altitude for? There are corrections you can apply based on meteorological models that allow you to convert pressure altitude to true altitude, but they involve quite some math.

@DeltaLima, I'm creating an E6B app as a programming excercise.  I wanted it to be mathematically true to the paper versions.  Determining the location of the individual tick marks on each scale sometimes requires a little reverse-engineering.  Knowing the precise formula used by manual E6B's (even if they weren't the most accurate formula) would help me recreate the most consistent experience.

I don't know the approximation used by E6B, but I do know the official mathematical model used by ICAO. Would that help?

Couldn't hurt, if nothing else it would be an educational opportunity for me. Thank you.

I encountered a similar question that goes as follows:
Q: Given the following information, what is the true altitude? (rounded to the nearest 50 ft)
QNH: 983 hPa
Altitude: FL 85
Outside Air Temperature: ISA - 10°
The answers available are:
A:
 7900 ft
 9400 ft
 7300 ft
 7600 ft
The correct answer is 7300ft - can anyone explain how we get to the answer?
Thanks

Since the OP is interested in an exact equation, let me take a stab at deriving it (though it's probably late for a 2016 question).
Premise: how altimeters work
Altimeters take a QNH and a static pressure in input and spit out an altitude. In order to do that, they assume that both the QNH and the pressure are values within the International Standard Atmosphere (ISA). With this hypothesis, it's easy to show that the relation between QNH, pressure and altitude is:
$$h=\frac{T_0}{L}\left[\left(\frac{\mathrm{QNH}}{P}\right)^\frac{R_sL}{g}-\left(\frac{P}{P_0}\right)^\frac{R_sL}{g}\right]\tag{1}$$
where:

$h$: Altitude reading in meters
$P$: Air pressure in Pascal
$\mathrm{QNH}$: Altimeter setting in Pascal
$L$: Temperature lapse $=0.0065 \mathrm{~K/m}$
$T_0$: Standard temperature $=288.15 \mathrm{~K}$
$P_0$: Standard pressure $=101325 \mathrm{~Pa}$
$g$: Gravitational acceleration $\approx 9.81 \mathrm{~m/s}^2$
$R_s$: specific gas constant for dry air $\approx 287.058 \mathrm{~J \cdot kg^{−1}K^{−1}}$

That's however not only how altimeters work. This relation is general and that's how altitude, pressure and QNH always relate to each other in the ISA. For this purpose it's useful to solve for $P$ as well:
$$P=P_0\left[\left(\frac{\mathrm{QNH}}{P_0}\right)^\frac{R_sL}{g}-\frac{Lh}{T_0}\right]\tag{2}$$
ISA corrected for temperature
Sometimes, instead of using ISA, it's useful to use ISA +X, i.e. standard atmosphere with X°C of variation. X is sometimes expressed as $X=T_\mathrm{OAT}-T_\mathrm{ISA}$. For example if at sea level pressure is 29.92 inHg and temperature is 10°C, then $X=-5$
Equation 1 works also in ISA +X but it needs a slight correction:
$$h_{ISA~+X}=\frac{T_0+X}{L}\left[\left(\frac{\mathrm{QNH}}{P}\right)^\frac{R_sL}{g}-\left(\frac{P}{P_0}\right)^\frac{R_sL}{g}\right]\tag{3}$$
The problem
The problem at hand basically means using equation 2 to get the air pressure outside the aircraft, and replacing it back into equation 3 (with the appropriate temperature correction X). If one does the replacement, the 2 equations simplify nicely in the following relation:
$$h_{true} = h\cdot\left(1+\frac{X}{T_0}\right)\tag{4}$$
where we use $h$ to denote the altimeter reading and $h_{true}$, the altitude with the temperature correction. Note that this equation is exact, it's not an approximation. If we replace the actual value of $T_0$ we get
$$h_{true} = h + 0.00347h\cdot\left(T_\mathrm{OAT}-T_\mathrm{ISA}\right)\tag{5}$$
$$h_{true}\approx h+h\frac{4}{1000}\left(T_\mathrm{OAT}-T_\mathrm{ISA}\right)\tag{6}$$
Which is the equation that shows up in other answers. I'm not sure why $\frac{1}{T_0}=0.00347$ is approximated as 0.004 but I suppose it's to make it conservatively safer (since it approximates the true altitude in excess)
Solution
In order to solve the problem, all we need to compute is the temperature deviation X and replace it in equation 5. To do so, we first compute the Pressure Altitude with the following relation:
$$\mathrm{P.A.}=h+\frac{T_0}{L}\left[1-\left(\frac{\mathrm{QNH}}{P_0}\right)^\frac{R_sL}{g}\right]=3670\mathrm{~m}=12041\mathrm{~ft}\tag{7}$$
It follows that:
$$X=T_\mathrm{OAT}-T_\mathrm{ISA}=T_\mathrm{OAT}-\left(T_0-L\cdot\mathrm{P.A.}\right)=-11.145\mathrm{~C}\tag{8}$$
Finally:
$$h_{true} = h + 0.00347\cdot h\cdot X=12016.6\mathrm{~ft}\tag{9}$$
One final note: calling it "true" altitude is misleading because, it probably better approximates the actual altitude, but the true altitude may still be different. The assumption is that temperature decays linearly with altitude, which may not necessarily be the case

For more formulas (there is another page for SI units if preferred) see: https://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html

I would think the OAT in the example is what your on-board thermometer is showing, so at 12,500 ft, not at sea level. ISA is +15°C at sea level, but at 12,500 ft it is -9.8°C, so you are just -10 off ISA. Let's see. 4 ⋅ 12.5 ⋅ (−10) = −500, so the formulas match.
Note though, that this is still just an approximation. The temperature lapse rate might also differ and that would have to be taken into account as well. The more precise equation is also nonlinear.

This filled in a big missing piece... I wasn't accounting for the lapse rate and so I was thinking -35 off ISA rather than -10.  But as you said, this is an approximation.  Do you happen to know "the more precise equation?"  I'm trying to replicate the result obtained from the E6B which is slightly lower than 12,000.  I suspect we use "4" for convenience but the actual value is 4.xxx

@dazedandconfused, I've seen some calculations in simulators, but never took the time to truly understand them. In any case while the temperature lapse rate is considered constant in the troposphere (up to 36,000 ft), the pressure and density still decrease exponentially, so the calculation is rather complicated.

If you need to compute the temperature deviation from ISA you need to use the pressure altitude (12000ft), not the indicated altitude (12500ft)

True Altitude= P.A. + ( (4/1000)* P.A.* Temp. Dev.)
Where P.A.= Pressure Altitude
Temp. dev.= Temperature Deviation from IsA temperature at that level

Q: Given the following information, what is the true altitude? (rounded to the nearest 50 ft)
QNH: 983 hPa
Altitude: FL 85
Outside Air Temperature: ISA - 10°

1:Recon QNH altitude, considering lower than ISA SL pressure, you`d have to rotate the Kollsmanswindow down, and with that the altimeter reading. How much? 30ft (~8m) / mB (hPa) the difference to standard altimeter of 1013,25 hPa, is -30,25hPa. -30,25*30= -907,5ft.
Subtract this altitude from pressure alt, results in QNH Alt. 8500ft-907,5ft=7592,5ft.
With Std Alt and OAT (calculate using a 2°C/1000ft, at 8500ft) comes to -11°C, set in your Aviat (or similar), read above 7600ft QHH alt, true alt on the outer scale: 7300ft.

Wow, that's quite different from the answer that Jeppesen gave (see question).


text:
To which reference point on the aircraft is the altimeter reading relative to?
To which point of the aircraft is the altimeter reading referred to? Is it referred to the probe position or is it offset to read as if it was at the bottom of the fuselage or the landing gear? If it was a GA aircraft I suppose it wouldn't make a difference, but on something big as a 747 or an A380, should it be set at the same level as the probe, there would be quite a difference compared to the ground (maybe 20-30 feet?). If so, how is this handled?

Along similar lines, some aircraft GPS navigation systems have a setting (usually in the "ground maintenance menu") for an offset to account for the height difference between the GPS antenna (typically on top of the fuselage) and the desired reference point (e.g. the main wheels).

The placement of a probe for a pressure  altimeter has exactly the problem you mention, but they only need to be accurate within 75 feet, so placement is not a problem.
For large aircraft, they use a "radar altimeter" which uses an electromagnetic beam to measure the distance to the ground, the same way police radars measure car speed. Radar altimeters are accurate to within a couple of feet.

Nice and clear answer. Was really wondering how the feet to ground voice distance in the cockpit was measured.

Addition to answer, radio altimeters are calibrated to main landing gear, so it's 0 at touchdown. They usually don't display more than 2500 feet.


text:
How big is the influence of color on the stability of the boundary layer?
I'm not sure if this is the best place, or physics.SE would a better place. I guess the aerospace engineers might be on both.
In class we were talking about stability of the boundary layer, and the influence of the temperature on the boundary layer.
The governing equation is given by Wiki boundary layer:
$$ u{\partial u \over \partial x}+\upsilon{\partial u \over \partial y}=-{1\over \rho} {\partial p \over \partial x}+{\nu}{\partial^2 u\over \partial y^2} $$
If we evaluate this $y=0$, where both $u$ and $v$ are zero:
$${1\over \rho} {\partial p \over \partial x}={\nu}{\partial^2 u\over \partial y^2} $$
We then deduced that, since $\nu=f(T)$, the instabilitiy of a boundary layer is thus a function of the temperature.
One of us mentioned that this was the reason that the Pepsi-livery Concorde had a lower maximum operating speed, since it is not white, but blue:
This would increase the temperature at the wing, making the boundary layer less stable.
I did so a search, which brought me to this Concorde SST website, which says that the wings were kept white, to reduce the temperature of the wing, but because of the fuel temperature. As the wings are white, the effects we were talking about are not relevant anymore, but still it made me wonder.
Could the color of a surface have so much influence on the stability of the boundary layer?

Wikipedia claims that, "Concorde had livery restrictions; the majority of the surface had to be covered with a highly reflective white paint to avoid overheating the aluminium structure due to heating effects from supersonic flight at Mach 2. The white finish reduced the skin temperature by 6 to 11 degrees Celsius."

@ChrisW: This extreme heat is found only near the stagnation line. Over most of the wing high temperature posed no problem.

I'm curious about this.  The white color REFLECTS heat away, mostly solar heating in this case.  A dark color radiates heat better - this is why the SR-71 Blackbird is black: to radiate the heat of friction between the airframe and wings, and the atmosphere.  By the way, Happy Birthday to the Blackbird - today is the 50th anniversary of its first flight.  So with a supersonic vehicle, how do the designers choose - Black or White?

@SkipMiller I'm wondering the same thing. There's a very interesting discussion about it here.

@SkipMiller: To understand how the paint helps in shedding heat, you need to know its reflective characteristic in the infrared range. What is highly reflective at visual wavelengths might be highly absorbent in the infrared range, and vice versa.

@fooot: That link redirects to the airliners.net forum homepage.

@Sean it looks like the new link is here.

Technically, yes. Practically, no.
Boundary layer stability
You are right that boundary layer stability depends on viscosity, but it depends much more on the pressure gradient. Accelerating flow exhibits extreme stability: Even if there is movement orthogonal to the flow direction (due to Tollmien-Schlichting waves), their speed becomes an ever smaller fraction of flow speed in accelerating flow. This kind of flow is always found right after the stagnation point, when the suction area over the mid section of the wing accelerates flow into it.
Conversely, decelerating flow has lower stability: Now any crossflow becomes relatively stronger simply because flow speed is decreasing. This kind of flow is found aft of the suction peak, when the flow has to manage the long and arduous pressure rise to the trailing edge. It does not take long, and transition into turbulent flow happens.
On the Concorde at high angle of attack, lift was created by a strong vortex. The boundary layer became turbulent right after the leading edge.
In cruise, the flow would hit the leading edge at a small angle, so the speed distribution over the wing was less peaky. The long chord length and high flow speed would still cause an early transition, however, so most of the wing had turbulent flow. Indeed, cooling the wing would lengthen the laminar portion, and heating would shorten it, but the difference in friction drag would be small when looking at the full picture.
Air temperature over the wing
In supersonic flow you always have both a temperature boundary layer and a viscous boundary layer. Air is heated when compressed and cools when it expands, and in supersonic flow these effects become too large to be ignored. Also, friction in the boundary layer heats up the air near the wing's surface.
The stagnation temperature increase at Mach 2 is 173°C. The leading edge temperature can be expected to be around 120°C (standard atmosphere air temperature being at -56°C in FL 600). Past the stagnation point, the flow accelerates into a suction field in which air temperature drops below its ambient value. At the bottom of the boundary layer, however, the temperature stays almost as high as at the stagnation point due to frictional heating. In a turbulent boundary layer on a flat plate the frictional effect would increase temperature by 152°C. Without the pressure distribution over the wing I cannot give a value for the absolute temperature, though.
Due to heat transfer with the wing, the air is cooled over much of the wing in the vicinity of the wing's surface. A dark coating will add a few degrees and result in a higher equilibrium temperature over much of the wing. A hotter boundary layer is thicker due to the expansion of air with temperature and will have higher viscosity. The most simple explanation why viscosity increases with temperature is that the mean molecular velocities, which are directly related to temperature, facilitate more transfer of shear forces between molecules. However, when it comes to wall friction, the effect of lower density with temperature outweighs the increase in viscosity, and wall friction is lower in a hotter boundary layer.
Thus, a heated boundary layer shows earlier transition and lower friction drag. The dark paint should actually help to lower the friction losses, and since the influence of temperature on the transition point is small, the change in friction due to lower density at the wall should dominate.
Maximum wing temperature
The maximum speed of the Concorde was limited by the stagnation temperature of air. If we assume that a switch from white to dark blue paint increases the equilibrium temperature of the wing leading edge by 10°C (which is not unreasonable), this increase could be compensated by a reduction of the maximum flight Mach number from 2.0 to 1.94. A restriction of the Pepsi Concorde to Mach 1.7 seems to me to be the result of extreme (and unreasonable) caution.
My verdict
I did not run the numbers, but I would expect that a different paint job would not change wing temperature much. At the leading edge and over much of the wing it adds a little to aerodynamic heating. A warmer wing would translate into less friction, but the difference is extremely small.
On the ground, in the absence of the strong convective heat transfer during supersonic flight, the temperature difference between white and dark wings is much more pronounced (in the order of 30°C to 50°C), and in this case a white wing is very effective in keeping the fuel temperature down and avoiding gas bubbles in the fuel line.

"While a heated boundary layer shows earlier transition, it also shows lower friction drag." , could you explain a bit more about why this is the case?

@ROIMaison: I needed to brush up on my knowledge, so it took a while ... I forgot how much friction will heat up the boundary layer at the wing.

Article on Concorde extreme temperatures (for a civil airplane), material, dilatation, paint...